I made tomato sauce yesterday but looking at the bottles today it has separated, can I re cook it to hopefully thicken it? I made it with fresh tomatoes.
Simple tomato sauces (type "coulis") often separate, as fresh tomatos contain a lot of water. We've had it with such sauces prepared for freezing, and used the sauce as a basis, which worked perfectly well.
You can recook it, but depending on how much liquid there is, that can require a lot of time: you'll have to evaporate must of the (separated) liquid. The taste will change (more 'caramelised', 'cooked' taste): longer cooking and higher temperatures at the end due to the higher concentration of dissolved matter.
Or you can skim off the liquid that separated.
To be safe, in both cases you'll have to sterilise your sauce again using the full cooking time.
But why bother recooking it if the taste was correct: such separation is perfectly normal in sauces that are not very concentrated and commercial tomato sauces have additives to avoid this separation (added ingredients like carrots help, and so does some starch, and there are others, look at the bottle). And if you use the sauce as a basis, you might cook it again at that stage.
The above assumes you did cook the sauce according to instruction regarding conservation (so long enough to sterilise the sauce correctly).